<template>
  <div>
    <div class="scroll-box" ref="scrollBox">
      <div ref="scrollItem" class="item" :class="{ active: currentIndex == index }" v-for="(item, index) in 10" :key="index">
        {{ index }} </div>
    </div>

    <div class="tips">点击加减号改变索引再点击启动按钮，观察滚动！！！</div>

    <div>当前索引是：{{ currentIndex }}</div>
    <div class="input-box">
      <div class="action">
        <div class="item" @click="reduce">-</div>
        <div class="item num">{{ currentIndex }}</div>
        <div class="item" @click="add">+</div>
      </div>
      <div class="start" @click="start">启动</div>
    </div>
  </div>
</template><script>
export default {
  data() {
    return {
      currentIndex: 0,
    };
  },
  methods: {
    add() {
      this.currentIndex++;
    },
    reduce() {
      this.currentIndex--;
    },
    start() {
      /**
       * 1)先让选中的元素滚到可视区域的最左边 scrollLeft
       * 2)接着向右移动容器一半的距离 containWidth / 2
       * 3)最后向左移动item一半的距离 offsetWidth / 2
       */
      let lastSpot = this.$refs.scrollBox.scrollLeft;
      const nextSpace = 7; //每次移动距离
      let scrollItemTimer = setInterval(() => {
        this.$nextTick(() => {
          let offsetWidth =
            this.$refs.scrollItem[this.currentIndex].offsetWidth; //item
          let scrollLeft = this.$refs.scrollItem[this.currentIndex].offsetLeft; //选中的元素滚到可视区域的最左边
          const containWidth = this.$refs.scrollBox.offsetWidth; //容器的宽度
          let resultSpot = scrollLeft + offsetWidth / 2 - containWidth / 2; //最终要停留的点
          if (Math.abs(lastSpot - resultSpot) < nextSpace) {
            clearInterval(scrollItemTimer);
          }
          if (resultSpot >= lastSpot) {
            lastSpot = lastSpot + nextSpace;
          } else {
            lastSpot = lastSpot - nextSpace;
          }
          this.$refs.scrollBox.scrollTo(lastSpot, 0);
        });
      }, 15);
    },
  },
};
</script>
<style lang="scss" scoped>
.scroll-box {
  margin-top: 50px;
  width: 330px;
  height: 40px;
  background-color: #ddd;
  white-space: nowrap;
  overflow-y: hidden;
  overflow-x: scroll;

  .item {
    display: inline-block;
    height: 100%;
    width: 90px;
    border-radius: 5px;
    background-color: #ff0;
    margin-right: 10px;
    text-align: center;
    line-height: 40px;
  }
  .item:nth-last-of-type(1) {
    margin-right: 0;
  }
}
.active {
  background-color: #0ff !important;
}
.tips {
  margin-top: 30px;
  font-weight: 700;
  color: #1989fa;
}
.input-box {
  width: 100%;
  display: flex;
  align-items: center;
  .action {
    display: flex;
    align-items: center;
    .item {
      width: 40px;
      height: 40px;
      background-color: #07c160;
      color: #fff;
      text-align: center;
      line-height: 40px;
      border-radius: 10px;
      margin-right: 5px;
    }
    .item.num {
      background-color: #ff976a;
    }
  }
  .start {
    width: 80px;
    height: 40px;
    background-color: #1989fa;
    color: #fff;
    margin-left: 100px;
    text-align: center;
    line-height: 40px;
    border-radius: 10px;
  }
}
</style>
